Your doubts are justified. Twitter does not guarantee that it is providing NodeXL with all tweets that match the search term, or that the tweets it provides on Monday will be the same tweets it provides on Tuesday. In fact, they explicitly state that the opposite
is true:

"...it's important to know that the Search API is focused on relevance and not completeness. This means that some Tweets and users may be missing from search results." (From
https://dev.twitter.com/docs/using-search )